1. Field of the Invention
The present invention relates to a substrate for an electron source which is to be used for forming the electron source, the electron source and an image forming apparatus in which the substrate has been used, and manufacturing method thereof.
2. Related Background Art
Conventionally, as an electron emission device, generally two kinds respectively using thermoelectronic emission device and cold cathode emission device are known. There are field emission type (hereinafter referred to as an FE type), metal/insulation layer/metal type (hereinafter referred to as MIM type) and surface conduction electron emission device, etc. for cold cathode electron emission device. As examples of the FE type, those which have been disclosed in W. P. Dyke and W. W. Dolan, “Field emission,” Advance in Electron Physics, 8,89 (1956) or C. A. Spindt, “Physical Properties of Thin-Film Field Emission Cathodes with Molybdenium Cones,” J. Appl. Phys., 47,5248 (1976), etc. are known. As examples of the MIM type, those which are disclosed in C. A. Mead, “Operation of Tunnel-Emission Devices,” J Apply. Phys., 32,646 (1961), etc. are known. As examples for the surface conduction electron emission device type, there are those which have been disclosed in M. I. Elinson, Radio Eng. Electron Phys., 10, 1290, (1965), etc. The surface conduction electron emission device is to utilize phenomena giving rise to the electron emission by making a current flow in parallel with the film surface at a small area of a film formed on a substrate. For this surface conduction electron emission device, the one utilizing SnO2 film by aforementioned Elinson et al, the one involving Au film [G. Dittmer, Thin Solid Films, 9,317 (1972)], the one involving In2O3/SnO2 film [M. Hartwell and C. G. Fonstad, IEEE Trans. ED Conf.” 519 (1975)], and the one involving carbon film [Hisashi Araki, et al, Shinku, vol. 26, the first issue, page 22 (1983)], etc. have been reported.
For the purpose of holding the electron source, which has been configured by a plurality of electron emission devices such as those described above having been disposed on the substrate, inside an enclosure whose interior portion has been held vacuum, and of using the electron source, it is necessary to implement junction between the electron source and the enclosure and other members. This junction is commonly implemented with flit glass by heating and melt-contacting. The heating temperature at this time is typically around 400 to 500° C., and the time period depends on the sizes, etc. of the enclosure or the like, around ten minutes to one hour is typical.
Incidentally, as quality for the enclosure, soda-lime glass is preferably used from the point of view that joint is implemented easily and without fail, and is comparatively low-cost with flit glass. In addition, high strain point glass, which distortion point has been raised with a part of Na having been replaced with K, can also be preferably used since its flit connection is easy. In addition, as concerns the substrate of the above-described electron source, in terms of its quality, similarly, soda-lime glass, or the above-described high strain point glass is preferably used from the point of view of their certainty of junction with the enclosure.
In the above-described soda-lime glass, as component thereof, an alkali metal element, especially Na is contained in large quantity as Na2O. Since the Na element easily gives rise to diffusion due to heat, when it is exposed to a high temperature during a processing, Na is diffused into respective members having been formed on the soda-lime glass, especially the member configuring the electron emission device, giving rise to changes in its features in some cases.
In addition, in case of using the aforementioend high strain point glass as the substrate of the electron source, the above-described influence due to Na is relieved to some extent according to a cut in contained quantity of Na, but nevertheless, it has been found out that similar problems takes place.
As means for reducing said Na's influence, in Japanese Patent Application Laid-Open No. 10-241550 specification, for example, EP-A-850892 specification, disclosed is a substrate for forming the electron source wherein density of the said contained Na in the surface layer region at the party where at least the electron emission device of the substrate containing Na is disposed has become smaller than the other regions, and moreover, the substrate for forming the electron source comprising a layer containing phosphorus. And on the other hand, the substrate on which the electron source is formed normally comprises insulating materials, and therefore, in the case where driving takes place under circumstances that a high voltage to be used for the purpose of causing electron emission has been applied, charge-up phenomena take place in the portion where the substrate is exposed, and in the case where no measures have not been taken whatsoever, it could become impossible to implement long-term stable drive, or the tracks of electrons emitted from the electron source will be disturbed, thus the electron emission features could change according to the lapse of time.
As means for reducing the influence by said charge-up, in U.S. Pat. No. 4,954,744 specification, for example, or Japanese Patent Application Laid-Open No. 8-180801 specification, it has been disclosed that the substrate surface or the electron emission device surface is covered by a charging prevention film comprising sheet resistance of 108 to 1010 Ω/□.